System for tomographic determination of the power distribution in electron beams

ABSTRACT

A tomographic technique for measuring the current density distribution in electron beams using electron beam profile data acquired from a modified Faraday cup to create an image of the current density in high and low power beams. The modified Faraday cup includes a narrow slit and is rotated by a stepper motor and can be moved in the x, y and z directions. The beam is swept across the slit perpendicular thereto and controlled by deflection coils, and the slit rotated such that waveforms are taken every few degrees form 0° to 360° and the waveforms are recorded by a digitizing storage oscilloscope. Two-dimensional and three-dimensional images of the current density distribution in the beam can be reconstructed by computer tomography from this information, providing quantitative information about the beam focus and alignment.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

This is a division of application Ser. No. 07/996,892 filed Dec. 28,1992 now U.S. Pat. No. 5,382,895.

BACKGROUND OF THE INVENTION

The present invention relates to the measurement of the current densitydistribution in electron and ion beams, particularly to a techniqueusing a modified Faraday cup to create an image of the current densityof such beams, and more particularly to a system and method using arotating modified Faraday cup in conjunction with computer tomography todetermine the current density distribution in electron and ion beams.

Over the years, various apparatus have been developed for determiningvarious characteristics of electron and ion beams, such as the beamconfiguration, diameter, energy peak, current density, spot size andedge width, etc. These prior approaches are exemplified by U.S. Pat.Nos. 4,336,597 issued Jun. 22, 1982 to T. Okubo et al.; 4,480,220 issuedOct. 30, 1984 to S. Isakozawa et al.; 4,629,975 issued Dec. 16, 1986 toR. Fliorito et al.; and 4,675,528 issued Jun. 23, 1987.

Electron beam machines have found wide application, particularly in thefield of welding, surface modification, x-ray generation, electron beamlithography, electron microscopy, etc. With these applications has comethe need for precise control of the beam focus and beam alignment, aswell as a particular need for determining the power distribution inelectron beams.

Reproducible electron beam processing can be made independent of themachine or the operator if the beam power distribution can be preciselycontrolled. Convention methods for setting the power distribution relyon the welding operator to visually focus the beam on a secondarytarget. The operator views the visible radiation or intensity of lightgiven off rather than directly measuring the power distribution of thebeam. This prior method is inherently imprecise, requiring significantoperator experience and judgment to set the beam focus consistently. Asreadily seen, each operator may set the machine parameters differentlydue to each one's visual interpretation.

The current density distribution is influenced by many variables,including the filament design, focus setting, work distance, beamcurrent, accelerating voltage, vacuum level, and filament alignment.Variations in these parameters may result in variations in the currentdensity distribution of the beam, which can have a significant effect onthe weld penetration, weld width, and surface quality of electron beamwelded materials. Thus, it is seen that the conventional methods forsetting the power distribution of electron beam machines has not beenfully satisfactory. Thus, quantitative diagnostic methods such as therotating wire device, the pinhole devices, and the modified Faraday cup(MFC) have been developed to more accurately determine the currentdensity distribution and thereby more accurate control of the beam focusconditions. The Faraday cup approach is exemplified by U.S. Pat. Nos.4,608,493 issued Aug. 26, 1986 to Y. Hayafuji; 4,703,256 issued Oct. 27,1987 to Y. Hayafuji; and 5,103,161 issued Apr. 7, 1992 to J. M. Bogaty.

The rotating wire device operates by scanning a thin electricallyconductive wire through the beam to sample the beam current. This earlydiagnostic method provided a means to measure the diameter of the beam,however, the accuracy of this method was limited by poor heatdissipation from the wire and difficulties associated with collectingback scattered and secondary electrons.

The pinhole devices provided another method for analyzing the spatialdistribution of current of non-uniform electron beams using a pinholeaperture to sample the current density. By rastering the beam over apinhole, current density measurements can be acquired at regularlyspaced intervals throughout the cross-section of the beam. Therefore,the current density, and thus the power density distribution, can bedetermined without making any assumptions about the beam symmetry.Although this method has the ability to map the spatial distribution ofpower in the beam, it tends to have a high signal-to-noise ratio due tothe fact that only a small percentage of the beam is transmitted throughthe pinhole, and inaccuracies caused by damage to the pinhole whileacquiring data are not correctable.

The modified Faraday cup (MFC) devices involve sweeping the electronbeam across a narrow slit. The major limitation of the MFC method isthat the beam is assumed to be radially symmetric with a circularcross-section section in order to measure the current densitydistribution with a single scan.

When a beam with a radially symmetric Gaussian current densitydistribution is integrated along one dimension, as is done by the slit,the result is a one dimensional Gaussian waveform which is a goodindication of the quality of the beam. However, when an irregularlydistributed (non-uniform) beam current is similarly integrated, theresult is an irregularly shaped waveform which, by itself, tells littleabout the beam's power distribution. Such non-uniform beams tend to takeon an elliptical or irregular cross-section power distribution, and tendto produce non-symmetric welds or processing conditions. The geometricshape of these welds varies with the orientation of the beam withrespect to the weld direction, and an accurate method for measuring thecurrent density distribution is required in order to precisely controlthe welding process. Fortunately, a number of measured waveforms takenat different slit angles can be used to reconstruct the beam currentdensity distribution using computed tomography (CT) algorithms developedfor the non-destructive evaluation of solid objects, thus providinginformation about non-symmetric beam distributions.

Computed tomography imaging has been developed in recent years toreconstruct the interior structure of objects using x-ray data, asdescribed in "Computerized Tomography Reconstruction Technologies",Energy and Technology Review, November-December 1990, S. G. Azevedo etal., UCRL-52000-90-11.12, pp. 18-34. In this method an object is scannedusing an x-ray source and a detector or a detector array. Detectedx-rays follow a line connecting the x-ray source and the detector. Thisray trace is analogous to the narrow slit on the MFC. The detected x-rayintensity will vary depending on the densities of materials in the path,just as the current measured through the MFC will depend on the currentdensity along the slit. The waveform resulting from a scan is called aprojection. The object is rotated and scanned at angles between 0-180degrees, as scanning more than 180 degrees produces redundant data. Theprojections are processed and used to reconstruct the beam currentdensity distribution. The greater the number of angles, the greater theaccuracy of the reconstruction. In reconstructions where fine detail ofthe solid object is required, increments of one degree or less may beused.

In view of the need to provide an approach for measuring the currentdensity distribution in electron beams, it has been recognized that bycombining a modified Faraday cup with the computer tomographic techniqueand rotating the Faraday cup, a tomographic determination of the powerdistribution in electron beams could be obtained, thereby resulting inthe present invention. The modification of the Faraday cup (MFC)involves rotating the cup in a selected sequence, such as by a steppermotor which allows for repeated rotation of the MFC and reorientation ofthe MFC slit angle, with deflection coils used in sweeping the beamacross the slit, thereby producing beam current waveforms which aremeasured with a current viewing resistor and recorded with a device suchas a digital storage oscilloscope, whereafter the waveforms arereconstructed utilizing computer tomography to produce a surface plot ofthe power density distribution of the electron beam.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a technique fordetermining the power distribution in electron or ion beams.

A further object of the invention is to provide a method and apparatusfor determining the profile data of an electron or ion beam.

A further object of the invention is to provide a method for measuringelectron beam current density using a rotating Faraday cup and acomputer tomographic technique.

A further object of the invention is to provide an apparatus fordetermining electron beam current density which includes a rotatingmodified Faraday cup having a slit across which the beam is directed ina controlled manner for producing output signals which can be used forvarious purposes including computer tomographic reconstruction showingthe power distribution of the beam.

Another object of the invention is to provide a modified Faraday cuphaving means for rotating same in a selected, controlled manner.

Another object of the invention is to provide a computer tomographictechnique to measure current density distribution in electron beams.

Another object of the invention is to provide a computer tomographictechnique to create an image of the current density in low and highpower beams using beam profile data acquired from a rotatable modifiedFaraday cup.

Another object of the invention is to provide a method for acquiringelectron beam profile data by sweeping the beam across a narrow slit ina Faraday cup, with the slit being repeatably rotated by selectiveamounts and the beam current waveforms being recorded at regularlyspaced angles.

Other objects and advantages will become apparent from the followingdescription and accompanying drawings which serve to explain theprinciples of the invention. Basically, the overall invention involves acomputer tomographic technique to measure the current densitydistribution in an electron beam which is swept across a narrow slit ina Faraday cup which is rotated in a stepped arrangement such that thebeam waveform is recorded at regularly spaced angles by a digitizingstorage oscilloscope, the recorded waveforms providing the input for thecomputer tomographic technique. The invention uses a rotatable modifiedFaraday cup incorporating tungsten slit blocks machined with an includedangle, such as 10°, that face away from the beam, and the currentpassing through the tungsten-formed slit and into an electricallyinsulated Faraday cup is measured via a current viewing resistor andrecorded with a digital storage oscilloscope to provide a beam profile.The modified Faraday cup (MFC) is rotated by a stepper motor that allowsfor repeated rotation of the MFC and reorientation of the slit angle androtation of the scan direction of the beam. The MFC is rotated andstopped at selected angles (degrees of rotation) to provide beam currentwaveforms taken over a series of predetermined angles to provide a beamprofile.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate an embodiment of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a schematic illustration of a data acquisition system foracquiring electron beam profile data.

FIGS. 2a and 2b illustrate embodiment of ribbon and hairpin filamentsused in the apparatus of FIG. 1.

FIGS. 3a-3c illustrate a top, partial cross-section, and partialexploded views of a modified Faraday cup (MFC) utilized in the FIG. 1apparatus.

FIG. 4 is an illustration of a current distribution passing over thethin slit in the MFC of the FIG. 1 apparatus.

FIG. 5 is a graph of a single scan electron beam profile showing thevoltage drop across the current viewing resistor as a function of scantime.

FIG. 6 illustrates a sinogram showing fifty beam profiles taken at 3.6°increments from radial scans back and forth across the beam between 0°and 176.4° which covers 360° of the beam after opposing beam profileswere added together.

FIG. 7 illustrates a tomographic reconstruction of the power densitydistribution as a surface plot for an 8 mA, 80 kV, ribbon-filament beammade in accordance with the present invention.

FIG. 8 illustrates a tomographic reconstruction as a contour plot forthe same beam utilized in FIG. 7, as produced by the invention.

FIG. 9 is a three-dimensional plot of the power distribution near thebeam waist of an 8 mA, 100 kV, beam produced by a ribbon filament.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is broadly directed to a computer tomography (CT)diagnostic technique for high resolution imaging of the current densitydistribution in electron beams. The invention may also be utilized forpower distribution determination in ion beams. This technique measuresthe electron beam (EB) profile by integrating the current density of thebeam along a thin slit, and CT reconstructs the current densitydistribution from multiple profiles taken at equally spaced anglesaround the beam. By using CT imaging, the current density distributionof non-circular and irregular electron beams can be quantified in orderto characterize the quality of the electron optical system and thequality of the electron beam.

More specifically, this invention uses electron beam profile dataacquired from a modified Faraday cup (MFC) to create an image of thecurrent density in high and low power beams. The beam profile data isacquired by sweeping the beam across a narrow slit in the MFC. The beamcurrent, integrated along the axis of the slit, is captured by the MFCand its waveform is recorded by a digitizing storage oscilloscope. Theslit is repeatedly rotated and beam current waveforms are recorded atregularly spaced angles (degrees of rotation). A two-dimensional imageof the beam's current density distribution in the plane of the rotatedslit is reconstructed via computer tomography from this information,providing quantitative information about the beam focus and alignment.

As set forth above, CT imaging has been previously developed toreconstruct the interior structures of objects using x-ray data, whereinan x-ray trace is analogous to the narrow slit on the MFC, and the x-rayintensity will vary depending on the densities of materials in the path,just as the current measured through the MFC will depend on the currentdensity along the slit. The waveform resulting from a scan is called aprojection. In the prior CT x-ray imaging techniques, the object wasrotated and scanned at angles between 0-180 degrees. Similarly in thisinvention the MFC is rotated from 0° to 180° in a repeated manner andthe beam is swept back and forth across the slit at each desired degreeof rotation. These projections are processed and used to reconstruct thebeam current density distribution. The greater the number of angles(scans during rotation) the greater the accuracy of the reconstruction.For more detailed information relative to the CT technology and the usethereof in the experimental verification of the present invention, seethe article entitled "A System For The Tomographic Determination Of ThePower Distribution In Electron Beams", A. Teruya et al., Proceedings ofthe Conference on the Laser and the Electron Beam in Welding, Cuttingand Surface Treatment State of the Art 1991, published 1992 by BakishMaterials Corporation, pp. 125-140.

In further investigation involving this invention, sharp focused anddefocused multi-kilowatt electron beams produced by hairpin and ribbonfilaments were tested using the MFC and CT imaging. These tests showedthat significant differences exist between the power densitydistribution produced by these different filament shapes. The results ofthis investigation further showed the influence that beam current andfocus conditions have on the peak power density and the power densitydistribution for both hairpin and ribbon filaments, such as used inelectron beam welding machines. This information can be used foraccurate transfer of welding parameters to different machines, improvingthe understanding of electron beam/material interactions, and generatingtailored power density distributions for improved electron beam weldingand surface treating of materials. For further details relative to theinvestigation on the effect of the different filament shapes on thepower density distribution of electron beams using the presentinvention, see "Tomographic Imaging of Non-Circular and IrregularElectron Beam Power Density Distributions", J. W. Elmer et al.,UCRL-JC-111341, dated August 1992.

FIG. 1 schematically illustrates an embodiment of the modified Faradaycup (MFC) system of the present invention for taking electron beamprofile data; with the details of the MFC being illustrated in FIGS.3a-3c. Basically, the FIG. 1 system involves three (3) interconnectedcomponents or systems: an electron beam gun generally indicated at 10, arotatable/movable MFC assembly generally indicated at 11, and a controland data acquisition system generally indicated at 12. System 12functions to control elements of the gun 10 and the MFC assembly 11 aswell as storing the acquired data.

The electron beam gun 10, such as may be used in a welding machine,basically comprises a filament 13, cathode 14, anode 15, alignment coil16, magnetic lens 17, and deflection coils 18. The filament 13 may be ofany desired cathode configuration, such as the ribbon type shown in FIG.2a and indicated at 13', or of the hairpin type shown in FIG. 2b, andindicated at 13". The various components of the gun 10 and details ofthe filaments 13' and 13" are known in the art and do not constitutepart of this invention. Thus, further description relative to theconstruction thereof is believed unnecessary to enable an understandingof the present invention, except that the deflection coils 18 areconnected so as to be controlled by system 12 to deflect an electronbeam produced by gun 10 and indicated at 19 in x and y directions asindicated. The beam 19 is moved via deflection coils 18 to sweep acrossa slit in the MFC 11 as indicated by the double-painted arrow.

The rotatable/movable MFC assembly 11 comprises an MFC 20, having a slittherein indicated at 21, mounted on a support 22 connected to arotatable member or stage 23 including a stepper motor 24 mounted on amovable assembly 25 composed of x, y and z translation stages, providingcapability of movement in the x, y, and z directions as indicated byarrows. By mounting the MFC 20 onto the stage 23 and stepper motor 24,such allows for controlled, repeated rotation of the MFC 20 andreorientation of slit 21 angle, described hereinafter. The z-transitionstage allows the MFC 20 to be positioned above and below the point ofbest focus. The x-y translation stages are used to provide horizontalmovement for beam 19 alignment purposes. Beam current waveforms aretaken with the MFC 20 and slit 21 being stationary by sweeping the beam19 perpendicularly across the slit 21 via the beam deflection coils 18.

MFC 20, as illustrated in detail in FIGS. 3a-3c, comprises a cylindricalbody or housing 26 having interconnected therein an internal cavity 27,an open end cavity 28 and a slit 29; a pair of blocks 30 and 31 havingtapered or beveled surfaces 30' and 31' and forming a slit 32 therebetween are mounted in internal cavity 27 via a pair of slit clamps 33,only one shown, and set screws 34 secured to housing 26, such that slit32 is in alignment with slit 21 formed in a plate 35 mounted on housing26, and with slit 29 in the housing; and a Faraday cup assemblygenerally indicated at 36, illustrated in FIG. 3c, is adapted to bemounted in open end cavity 28 and retained therein by a retainer plug37, having a central opening 37', which is mounted in an enlarged end28' of cavity 28. Thermal expansion of slit 32 between blocks 30 and 31may be further minimized by placing a copper housing around MFC 20 toabsorb the majority of the beam's energy. The Faraday cup assembly 36includes a Faraday cup 38, an insulator 39 having openings 40 and 41therein, a cap 42 having a slot 43 therein, and an insulator 44 having aslot 45 therein. Faraday cup 38, cap 42 and insulator 44 are positionedin opening 40 in insulator 39 and insulator 39 is retained in opening 28of housing 26 by plug 37. An electrical lead or cable 46 secured at oneend to Faraday cup 38 extends through openings 41 and 51' in insulator39 and through passage 51 in housing 26 and is connected to a currentviewing resistor 47, and to a digital storage oscilloscope in system 12(see FIG. 1). The current collected by the cup 38 produces an outputsignal indicated at 46' in FIG. 3c that passes through the cable 46 tothe outside of a vacuum chamber, not shown, containing housing 26 via avacuum, feedthrough and sinks to a common ground as indicated at 48through the current viewing resistor 47. The voltage across the resistor47 is measured by the digital storage oscilloscope of system 12, whichstores the data on a floppy disk. Housing 26 is electrically connectedto the common ground as indicated at 49, while blocks 30 and 31 areconnected to common ground as indicated at 50. While grounds 48, 49 and50 are individually identified, they are common in actual practice.

By way of example, housing or body 26 may be constructed of a highthermal conductivity material such as copper, nickel, silver, or similarmetals and alloys, 75 mm in diameter and 75 mm high, with the opening 27having a cross-section of 21/4 inch and height of 1/2 inch, with theopening 28 having a 1 inch diameter and length of 21/8 inch, and openend 28' having a 11/2 inch diameter and depth of 1/8 inch; and with slit29 having a length of 1 inch and width of about 1/4 inch. The slitblocks 30 and 31 are constructed of refractory metals or alloys such astungsten, tantalum, or tungsten-rhenium, phenium with a length of 2inches, width of 1 inch, and height of 3/8 inch, with the tapers orbevels 30' and 31' being at a 10 degree angle, and with the slit 32formed there between having a width of 0.002-0.005 inch. The slit clamp33 is formed from the same material as the slit blocks, with a thicknessof 3 mm (1/8 inch), and the sets screws 34 may be stainless steel orsteel and hold the tungsten blocks 30 and 31 against the copper housing26 to increase heat transfer from the blocks to the housing. The Faradaycup 38 is constructed of copper and is 25 mm in diameter and 35 mm high,the insulators 39 and 44 are constructed of a high melting point,electrically insulating material, such as a ceramic with the plug 37 andcup 42 being constructed of the same material as the Faraday cup(copper). The plate 35 is made of the same material as the slit blocks,such as tantalum, tungsten, or tungsten-rhenium, with the same diameteras housing 26, a thickness of 1/4 inch, with the slit 21 having a lengthof 1 inch and width of 1/4 inch.

By way of example the resistor 47 may be 100 ohms. The grounding wires,indicated at 49 and 50 between the tungsten slit blocks, the copper bodyand the interior of the vacuum chamber, in which the body is located,serve to prevent charge build-up. The slots 43 and 45 in cup 42 andinsulator 44 align with the slit 32 and prevents the loss of signal dueto reflected electrons. The tantalum plate 35 serves to protect the topof the body or housing 26 from the electron beam 19. The tungsten slitblocks 30 and 31 are tapered beveled to avoid beam reflection into theFaraday cup 38. While the beveled or tapered surfaces 30' and 31' areexemplified as being at a 10 degree angle, the angle can be increased ordecreased by about 2-4 degrees, but must be sufficient to prevent beamreflection and/or secondary electrons and not so wide as to reduce thethickness of the blocks 30 and 31 adjacent the slit 32 which wouldresult in damage due to the heat generated by the electron beam passingthere through. The slit 32 while exemplified above as having a width of0.002-0.005 inch and a length of 1 inch may be greater or smallerdepending on the configuration and energy of the electron beam 19. Anarrower slit is more precise than a wider slit and the slit must have alength of at least the diameter or cross-section of the beam 19, but canbe longer than the beam diameter. A general ratio of slit width to beamdiameter is about 1 to 10. The MFC embodiment illustrated in FIGS. 1 and3a14 3c is designed for high power electron beam welding applicationsusing a voltage of 20 kV to 150 kV and current of 1 mA to 250 mA.However, the invention may be used on lower power beams down to 1 kV fortelevision, electron microscope (10's of kVs), and medical x-raygeneration applications, for example. With the lower power application,different materials, dimensions, etc. of the MFC may be utilized due tothe lower heat generation. Also, for higher power applications (up to300 kV and 1000 mA) the MFC 20 may be mounted in a liquid cooled housingto absorb heat from the beams.

The control and data acquisition system 12 of FIG. 1 includes a digitalstorage oscilloscope 52 into which the output signal 46' via lead orcable 46 is directed; a motor controller 53 electrically connected asindicated at 54 to stage 23 and stepper motor 24; an x-y coil deflectioncontroller 55 electrically connected as indicated at 56 and 57 todeflection coils 18 of electron beam gun 10; a function generator 58electrically connected to oscilloscope 52 and deflection controller 55as indicated at 59 and 60; and a computer 61 electrically connected tomotor controller 53 and deflection controller 55 as indicated at 62 and63. The computer 61 may be a personal computer (PC) such as an IBM ATComputer (PC/AT).

The MFC 20 is positioned on rotating stage 23 containing stepper motor24 which is controlled by motor controller 53 so as to provide selected,periodic, repeated rotation of the MFC and reorientation of the angle ofthe slit 32 with respect to the sweep of beam 19. The z-translationstage of moveable assembly 25, as indicated the legend z-control, allowsthe MFC 20 to be positioned above or below the estimated point of bestfocus, and translated through the point of best focus. The x-ytranslation stage of assembly 25 is used to provide horizontal movementfor beam alignment purposes. Beam current waveforms are taken with theMFC 20 and slit 21 stationary by sweeping the beam 19 across the slit 21with the beam deflection coils 18 which is controlled by deflectioncontroller 55. The degree of rotation of the MFC is dependent on thedesign of the stepper motor and the number of beam profiles desired,such that the MFC is rotated in repeated degree increments through arange of 0 to 180 degrees. The increments may be determined by thenumber of waveforms desired. For example, if 25 beam profiles aredesired for beam reconstruction then the waveforms would be taken at7.2° increments (180/25), and if 50 beam profiles are desired for beamreconstruction then the waveforms would be taken at 3.6° increments(180/50). In beam reconstruction, the greater number of beam profilesprovide for finer detail of the beam, and thus under certain conditions,increments of one degree or less may be obtained. For easiestreconstruction the beam 19 should sweep across slit 21 perpendicular tothe slit. However, while a perpendicular sweep of beam 19 across slit 21is preferred, a plus or minus of up to 90° from the perpendicular willin most instances provide an acceptable waveform, provided of coursethat the slit is of sufficient length to allow the diameter of theentire beam to be swept across the slit. However, an angular accuracy of±10° can be tolerated when the beam is swept perpendicular to the slit.This tolerance gets smaller as a function of the cosine of the angle;between the sweep direction and a line perpendicular to the slit. Wherethe beam sweep is not perpendicular to the slit and the slit is longenough and is uniform in width, and provided such is recognized, and theexact angle is known, the waveform and thus the resulting beam profilescan be compensated for during the computer tomographic reconstruction.

While the x, y, and z movements of the MFC are adjusted manually by theoperator, the MFC is rotated and the slit angle (degree of rotation) setby computer control via computer 61 and controllers 53 and 55 of system12. Software and hardware for the computer 61 have been developed whichallow the operator to specify an angle and let the computer rotate theMFC to the desired position. While in the set-up mode, the MFC can berotated at the finest angles available on the stepper motor. When inoperation mode, the MFC can only move sequentially to angles (degrees ofrotation) predesigned for beam profile reconstruction. At each setangle, the MFC and slit are stationary during the sweep of the beamperpendicularly across the slit.

Sweeping the beam 19 across the slit 21 is done using the beamdeflection coils 18 of the EB gun 10 and the external electronics ofsystem 12. The beam sweep is controlled with the function generator 58which provides a triangle wave with the desired period and amplitude.The x-y deflection controller 55, which was designed to performpolar-to-rectangular conversion, receives the triangle sweep signal 60from generator 58 and an angle value (degree of rotations) signal 63from the computer 61, and generates a pair of signals 56-57 for the x-ybeam deflection coils 18. In order to minimize the chance of damage tothe MFC, fast scan speeds (200 to 500 inch/sec.) for the beam deflectionare used, but they must also allow the capture of an adequate number ofdata points by the digitizing oscilloscope 53.

Data is taken by sweeping the beam 19 orthogonally across the slit 21and measuring the beam current passing through the slit using Faradaycup assembly 36. This integrated Slice of beam current is measured withthe current viewing resistor 47 and recorded with the digital storageoscilloscope 53 as the beam 19 moves across the slit 21, producing atime record referred to as a beam profile. Such a single scan electronbeam profile showing the voltage as a function of scan time isillustrated in FIG. 5. Knowing the beam sweep speed and the value of thecurrent viewing resistor, the integrated beam current is determined as afunction of position. These beam profiles are stored in oscilloscope 53.The data is later transferred to a conventional computer workstationcapable of running the software for processing and CT reconstruction ofthe beam profile. Such conventional stations may, for example, be a VAXor SUN station.

After the beam is focused, by the z-control of assembly 25, or byadjusting the focus coil current, the slit and direction of beam sweepare set perpendicular to each other by the x-y translation stage ofassembly 25 and the controllers 53 and 55. Once this is done, thecomputer controlled electronics maintains the perpendicular alignment ateach sweep angle (degree of rotation). With the triangle sweep signal 60applied to the x-y deflection coils 18, the beam 19 is turned on, giventime to stabilize and the waveform from the modified Faraday cup iscaptured on the oscilloscope 53. A number of Gaussian-like peaks arecaptured in each waveform record as the beam sweeps back and forth overthe slit.

The operation of the MFC technique is illustrated in FIG. 4. Here acurrent distribution is schematically shown to be moving in thex-direction at a constant travel speed S, over a slit of width Δx. Theamount of current, I_(S) (x), passing through this slit at any giventime is given by: ##EQU1## where J(x,y) is the current densitydistribution in the beam incident on the x,y surface.

The current density distribution of the electron beam may take on anumber of forms. Two common forms that have analytic descriptions arethe circular and elliptical Gaussian distributions. For a circularGaussian distribution, the current density is given by: ##EQU2##

In this expression, I_(B) is the total beam current and σ_(c) is thestandard deviation of the circular Gaussian beam. For an ellipticalGaussian distribution, the current density is given by a similarexpression: ##EQU3## where σ_(a) σ_(b) are the standard deviations ofthe major and minor axes of the elliptical Gaussian distributionrespectively, and x' and y' refer to the directions along the major andminor axes of the ellipse respectively. The coordinate transformation tothe primed axes represents a rotation through an angle, θ, with respectto the unprimed axes.

For both the circular and elliptical Gaussian beams operating at aconstant acceleration voltage, the power density distribution, P(x,y),is the product of the current density distribution and the voltage:

    P(x,y)=J(x,y) V.sub.a (kW/mm.sup.2),

where V_(a) is the accelerating voltage of the beam. Therefore, thepower density distribution of a circular Gaussian beam can be calculatedfrom a single sweep of the beam over the MFC to determine σ_(c).However, the power density distribution of an elliptical Gaussian beamhas two distribution parameters, σ_(a) and σ_(b), which cannot becalculated from a single scan, but can be calculated by tomographicreconstruction from multiple scans taken at different angles around thebeam. The distribution parameters σ_(a), σ_(b) and σ_(c) are oftenrelated to the diameter of the beam at 50% of the peak amplitude, whichis defined as the full width at half maximum amplitude, FWHM, of thebeam. The FWHM_(a), FWHM_(b) and FWHM_(c) parameters can be calculatedby multiplying σ_(a), σ_(b) and σ_(c) respectively, by a factor of 2.35.

Defocused electron beams and electron beams generated bytemperature-limited filaments frequently have complex power densitydistributions. These beams may be highly irregular and cannot bedescribed by analytic expressions. However, since computer tomographydoes not require J(x,y) to be described by an analytic expression inorder to measure its power distribution, CT imaging can be used toanalyze complex non-symmetric beams as easily as symmetric beams.

FIG. 5 shows an oscilloscope trace for a typical electron beam asmeasured by the MFC. The x-axis is plotted in μs, and represents theelapsed time as the beam travels over the slit. This axis can beconverted into beam position through the relationship: x=τ•S, where xrepresents the position of the beam, t is the scan time and S is thescan speed of the beam. The y-axis plots the voltage drop across thecurrent viewing resistor, and can be converted into slit current throughthe relationship: Vm(x)=I_(S) (x) R, where V_(m) (x) and I_(s) (x) arethe measured voltage drop and slit current respectively as functions ofx, and R is the resistance of the current viewing resistor.

The procedure for taking data during the experimental verification ofthe invention consisted of using the x and y table movements and the MFCrotation to align the MFC in the EB chamber so that the center of theslit was positioned directly below the dead-center position of theelectron beam. With the beam turned off, the function generator wasactivated so that the beam would be deflected to one side of the slit.Once the beam was turned on and the full beam current was established,the beam was scanned back-and-forth over the slit. Two beam profileswere captured by the oscilloscope as the beam crossed the slit andreturned to its initial position. The beam profiles were visuallyinspected on the oscilloscope prior to storing them as a permanentrecord on the disk. Once the data was recorded, the MFC was rotated bythe computer to its next position prior to taking the next profile. Thisprocess was repeated until the entire beam (360°) had been analyzed.

A typical tomographic run consisted of gathering 25 beam profiles takenat 7.2° increments through 180°. Some runs were made with higherrotational precision and consisted of 50 beam profiles taken at 3.6°increments as illustrated in FIG. 6.

There are several different techniques for reconstructing atwo-dimensional image from a set of projections (waveforms at a givensweep angle), a process known as backprojection. See A. C. Kak et al.,Principles of Computerized Tomographic Imaging, IEEE Press, New York,N.Y., 1988, pp. 60. A modification of this process has been developedwhich provides two and three-dimensional image and is a method ofbackprojecting of filtered projections. See above referenced documentUCRL-90-11.12 by H. E. Martz et al. This process for creating thebackprojected image consists of five steps: scaling, filtering,smearing, rotating, and adding.

Since each beam profile contains the integrated beam current, scaling isdone by dividing each data point in the profile by the number of dataangles to compensate for the number of times this data is added to thebackprojection. A filter within the frequency domain is then applied tothe projection. The Fast Fourier Transform (FFT) of the projection ismultiplied by a filter that emphasizes high frequency components. Theinverse FFT returns the filtered projection which is then smearedvertically to form a two-dimensional image where each row is thefiltered projection. This image is then rotated to the angle of themeasurement, and the rotated image is added to a summing image. As thisis done for more and more projection angles, the peaks in the profilesreinforce each other and become prominent, and the summing image becomesa representation of the density distribution. This image may then bedisplayed as a pseudo-color image, as a contour plot, or as a 3-drepresentation with the current density being represented in the thirddimension.

The waveform data is transferred to a conventional workstation computerand prepared for backprojection using a commercial image processingsoftware package. Uniformly long data arrays containing the selectedcurrent peaks at each sweep angle are extracted from the data recordsand combined to form an image called a "sinogram". The sinogram is thefirst step in the reconstruction and is the first chance to examine thequality of the entire data set. The waveforms for each angle form a rowin the sinogram. With 20 angles equally spaced at nine degree incrementsbetween 0 and 171° and 200 points in each waveform peak, the sinogramimage is 200 pixels wide and 20 pixels high. The term sinogram comesfrom the fact that a prominent feature not at the center of rotation ofthe image will appear in different positions in each row and seem toroam across the image in a sine wave pattern.

The areas under the waveform peaks for all angles in the sinogram arenormalized to remove the effects of time varying changes of magnitude ofthe beam current. It is also important to have all of the profiles ineach row of the sinogram aligned so the center of rotation is at thecenter of each row. Most CT systems have the x-ray source and detectorand the rotating stage permanently aligned so the center of rotation iswell known. Physical limitations make it almost impossible to align thecenter of the slit, the axis of the rotating stage, and the axis of theelectron beam with the precision and accuracy needed for tomographicreconstruction. Consequently, a method for finding an artificial "centerof rotation" is used.

Just as an object has a center of mass which is the point where theentire mass may be considered as concentrated, the beam currentdistribution will have a center of current density. For any arbitraryangle, a center of mass-type calculation will provide the position ofthe center of current density within a profile. Since this is the onlypoint that can be determined reliably for any profile, it is used as the"center of rotation" placed at the center of each row in the sinogram.

The MFC method acquires two peaks for every cycle of the functiongenerator as the beam scans one direction and then back to its startingpoint. Since the second peak is acquired when the sweep is returning,this peak represents data taken at an angle 180 degrees from the initialscan angle. Prior to tomographic analysis, the second peak is reversedand averaged with the first peak. This averaging reduces noise input tothe reconstruction.

The sinogram image produced with the image processing software package,such as a PV-Wave, is then transferred to a conventional workstationcomputer, such as a Sun workstation. Filtered backprojection software onthe workstation is used to produce tomograms (CT-reconstructions) fromthe sinogram data. Tomograms are then transferred back to the initialworkstation and image processing software for examination and finalprocessing.

As set forth above, tomographic analysis of the EB power distributionconsists of combining the individual beam profiles, such as shown inFIG. 5, into a "Sinogram" which is the first step in the reconstructionprocess and allows individual beam profiles to be qualitativelycompared. CT reconstruction if then performed on the sinogram in orderto reconstruct the power density distribution of the electron beam.

The beam profiles were analyzed using imaging software running on acomputer workstation in order to generate the sinogram data file thatwas used as the input file for the tomographic reconstruction. The firstprocessing step in tomographic reconstruction registered the centroid ofeach profile at the same temporal location using a center of mass typecalculation. This adjustment was necessary because the beam profile wasrandomly located in the data-file record. A second adjustment was madeto the beam profiles in order to normalize the area under each curve sothat all profiles have the same area. This adjustment is valid forconstant current beams and was necessary because of small changes in theslit width that are caused by erosion of the slit block or thermalexpansion related widening of the slit during repeated scans. The areasunder the curves typically required a ±5% normalization factor to bringthem to the same value.

Tomographic reconstruction was then performed on the beam profiles usinga filtered backprojection algorithm similar to NDE techniques. In thistechnique each profile is transformed into the frequency domain using aFast Fourier Transform (FFT). A ramp filter is then applied to emphasizehigh frequency components, and the filtered projection is returned tothe spatial domain. A two-dimensional image is created where each row isa replication of the filtered profile. Each of these images is rotatedto the angle at which the profile was acquired, and the sum of theserotated images is the reconstruction. The summing process emphasizes thecourse structure of the image, which appears in all profiles andbalances out the emphasis on high frequency components that are createdby the ramp filter. The final step in the reconstruction is to normalizeto the total power of the beam.

While a tomogram has the shape of the distribution, it must still bescaled to correspond to the units of current density or power density.This is done by normalizing the reconstruction so the integral of thereconstructed beam is equal to the beam current and the tomogram is inthe units of Amperes per pixel. In order to convert to units of Amperesper square meter the value for each pixel is divided by the area of apixel (the square of the distance corresponding to each point in theprofile). If the power distribution is required, the value of each pointis multiplied by the accelerating voltage.

During early experimental verification of this invention a comparisonwas made of the focusing ability of an experienced electron beam machineoperator with that of the MFC, as illustrated in FIG. 1, and whichclearly illustrated the advantages of the MFC method. The result of thiscomparison and other experiments relative to this invention are setforth in above-referenced article by A. Teruya et al. published byBakish Materials Corporation.

FIG. 6 illustrates a typical sinogram which appears as a series of beamprofiles stacked next to each other. This figure shows 50 beam profilestaken at 3.6° increments. The x-axis represents the scan time, which canbe converted to spatial distance knowing the beam scan speed; the y-axisplots the individual beam profiles from 0 to 180°; and z-axis representsthe measured voltage drop across the current viewing resistor. Thissinogram was taken from an 8 mA, 80 kV elliptical Gaussian beam wherethe individual beam profiles vary for different scan directions. Thisvariation leads to the peaked sinogram shape where the tall and narrowbeam profiles were taken while scanning at right angles to the minoraxis of the ellipse, and the short and wide beam profiles were takenwhile scanning at right angles to the major axis of the ellipse.

FIG. 7 shows a surface plot of the power distribution reconstructed fromthe sinogram in FIG. 6. The axes on this reconstruction have beenconverted to power density (kW/mm²) on the z-axis, and spatialcoordinates (mm) on the x and y axes. From this plot, the peak powerdensity, spatial power distribution, and degree of radial symmetry ofthe beam can be determined. Other useful tomographic reconstructionformats can be produced such as the contour plot of these same data,which is shown in FIG. 8. FIGS. 7 and 8 illustrate tomographicreconstruction of the power density distribution for an 8 mA, 80 kV,ribbon-filament beam based on 100 beam profiles, see FIG. 6, taken at3.6° increments from radial scans back and forth around the beam between0° and 176.4°.

FIG. 9 shows a three-dimensional plot of the power distribution near thebeam waist and consists of a series of tomographic images from an 8 mA,100 kV, ribbon-filament beam at 6.35 mm increments along the beampropagation axis. Here, the images are stacked above each other toillustrate the beam profile from -25.4 to +25.4 mm, where the 0 mmlocation refers to the point of sharpest focus. It is apparent that thedefocused beam has an elliptical shape and the focused beam has a nearlycircular shape.

Substantial, experimental efforts have been carried out for verificationof the invention relative to various beam configurations and energy, aswell as the effects of ribbon and hairpin filaments utilized in theelectron beam machines. For further information relative to theseexperiments see above-referenced UCRL-JC-111341 by J. W. Elmer et al.

The following sets forth the operational steps involved in: A) obtainingbeam waveforms via the apparatus illustrated in FIGS. 1 and 3a-c; and B)utilizing these beam waveforms via CT reconstruction to produce atomographic image of the beam.

A) Obtaining beam waveforms:

1. Set the beam to the desired focus setting.

2. Move the z-axis stage until the slit is at the desired height.

3. Rotate the cup to 0° measurement angle.

4. Adjust the x-y translation stage to align the beam with the slit.

5. Deflect the beam to one side by turning on the x-y deflection coil.

6. Turn on the beam and wait for full current to be established.

7. Raster the beam back and forth across the slit using the x-y coils.

8. Capture waveform and store.

9. Turn off the beam.

10. Turn off the x-y deflection coil.

11. Rotate cup to next measured angle.

12. Repeat steps 4-11.

B) CT reconstruction:

1. Read waveforms into imaging processing software.

2. Select waveform peaks and select a uniform section around the peakscenter of mass.

3. Create sinogram by placing all the peaks in a single array.

4. Create tomogram using the filtered backprojection technique.

5. Display.

It has thus been shown that computer tomographic (CT) imaging can beused as a high resolution method for measuring the power densitydistribution in high-power and low-power electron beams. This method canbe used to analyze the power density distribution of non-circular andirregular beams in order to determine the beam orientation, the shapefocus condition along any given scan direction, and the depth of fieldalong the beam propagation axis. When applied to welding, CT imaging canbe used to help reduce the amount of operator judgment required to focusand align the beam, improve the reproducibility of the welding process,assist with the modeling of electron beam/material interactions, andassist with the development of new electron guns and filament designsfor improved electron beam power density distributions.

It has also been shown that the present invention provides arotatable/movable modified Faraday cup assembly which can effectivelyprovide information relative to an electron beam which can be utilizeddirectly or in combination with CT techniques to provide a betterunderstanding of the beam energy, characteristics, etc.

While the description of the present invention has been primarilydirected to use of the invention for electron beams, particularlyhigh-power, high intensity multiple kilowatt (20 kV plus) electronbeams, it can be utilized with low-power (1 kV) beams, as well as foranalysis of ion beams. Thus, the invention is not limited only toelectron beam applications, such as used in welding machines, but has awide application for the analysis of any type of energy producing beams,such as the generation of x-rays or use in electron beam lithography.

Also, while particular embodiments, materials, parameters, have beenillustrated and/or described, the present invention is not limitedthereto. Modifications and changes will become apparent to those skilledin the art, and the scope of this invention is intended to cover such,with the invention to be limited only by the scope of the appendedclaims.

We claim:
 1. An apparatus for producing beam profiles from a beam ofenergy for use, such as in computer tomographic reconstruction andimaging of the beam of energy, comprising:means for producing a beam ofenergy; a modified Faraday cup assembly having a rotatable section andpositioned to be swept by said beam of energy produced by said beamproducing means; and control and data acquisition means operativelyconnected to said means for producing a beam of energy and to saidmodified Faraday cup assembly, said control and data acquisition meansbeing connected to control deflection of said beam of energy and tocontrol rotation of said rotatable section of said modified Faraday cupassembly.
 2. The apparatus of claim 1, wherein said means for producinga beam of energy includes at least one beam deflection coil, and whereinsaid control and data acquisition means includes electronic meansoperatively connected to said deflection coil for deflecting a beam ofenergy across said modified Faraday cup assembly in a controlled manner.3. The apparatus of claim 2, wherein said electronic means includes anx-y deflection controller connected to said deflection coil, a functiongenerator connected to said deflection controller, and a computerconnected to said deflection controller.
 4. The apparatus of claim 1,wherein said means for producing a beam of energy is an electron beammachine.
 5. The apparatus of claim 1, wherein said modified Faraday cupassembly additionally includes means for moving said Faraday cup in x, yand z directions for aligning said Faraday cup with said beam of energyand for focusing the beam of energy on said Faraday cup.
 6. Theapparatus of claim 5, wherein said rotatable section of said modifiedFaraday cup assembly includes a stepper motor, and wherein said controland data question means includes electronic means operatively connectedto said stepper motor controlling rotation of said section.
 7. Theapparatus of claim 6, wherein said electronic means includes a motorcontroller connected to said stepper motor, and a computer connected tosaid motor controller.
 8. The apparatus of claim 6, wherein saidmodified Faraday cup assembly includes a housing having an openingtherein, a Faraday cup positioned in said opening, and members forming aslit there between positioned in said opening and in alignment with saidFaraday cup such that energy from a beam of energy passes into saidopening, through said slit, and into said Faraday cup when a beam ofenergy is swept across the modified Faraday cup assembly.
 9. Theapparatus of claim 8, wherein said Faraday cup is electrically connectedto a storage oscilloscope of said control and data acquisition means viaa current viewing resistor.
 10. The apparatus of claim 8, wherein saidmembers forming a slit there between are constructed from materialselected from the group consisting of tungsten, tantalum,tungsten-rhenium and tantalum-tungsten alloy; each of said members beingprovided with a tapered surface on the adjacent sides thereof formingsaid slit.
 11. The apparatus of claim 8, additionally including a platehaving a slit therein positioned on said housing such that said slittherein is in alignment with said slit formed by said members.